In recent years, a cylinder for a core of a mold which is mounted on a die casting machine, injection molding machine or the like has been directly fixed on a body of the machine to meet the need for diminishing the total size and weight of the apparatus. As a consequence, the cylinder becomes heated to a high temperature by the heat transmitted from the body which is at a high temperature. This has resulted in using a U packing made of good heat-resistant fluoro-rubber as a sealing member for shutting off leakage between the cylinder case and a piston rod.
In addition, oils which might possibly cause a fire by leaking and coming in contact with a mold at a high temperature usually are not used as an operating fluid for driving the core of a mold such as described above. Instead, a water/glycol mixture containing about 95% of water is used.
However, a sealing member made of above-mentioned fluoro-rubber produces a chemical reaction in contact with water of the water-glycol mixture when it is heated to a high temperature with the result that the seal deteriorates early and a sealing failure occurs. Such a failure causes the operating fluid to leak through the clearance between the cylinder case and the piston rod to the outside. It is therefore necessary to replace frequently the sealing member before it deteriorates, a requirement which makes its maintenance troublesome.
In order to solve such a problem, a system has been proposed as disclosed in Japanese Utility Model Laid-Open Publication No. 64-25508. In that system, the cylinder comprises a cylinder case having a front wall on a front end, a piston slidably fitted into the cylinder case, a piston rod which is coupled to the piston and passes through a hole in the front wall of the cylinder case, and a sealing member attached to the periphery of the hole. An annular passage is defined in the front wall which surrounds the sealing member and a passage which feeds and discharges the fluid is formed in the cylinder case. This prevents deterioration of the seal and simplifies its maintenance.
In addition, a cylinder similar to that of the system as described above, for example, a cylinder as described in Japanese Utility Model Laid-Open Publication No. 61-119609 is known. In that structure a clearance is formed substantially over the full length of a cylinder, between a cylinder tube and a jacket, by enclosing an outside of the cylinder tube with the jacket. The heat from outside is shut off by feeding and discharging cooling water through the clearance.
In either of these fluid cylinder structures, although the heat transmitted through a cylinder case (cylinder tube) to a sealing member can be intercepted along its path, the heat transmitted through the piston to the sealing member cannot be shut off at all. Thus, when the end of this piston rod is connected to any component heated to a high temperature, the heat is directly transmitted through the piston rod to the sealing member which is then heated to a high temperature. The sealing member then deteriorates rapidly due to its contact with the water of the operating fluid and leakage of the fluid results. Such failure can be prevented only by frequent replacement of the sealing member which involves a troublesome and expensive maintenance procedure.